1. Field of the Invention
The present invention relates to a magnetic head that incorporates a sensor for detecting the presence or absence of a projection on a surface of a magnetic disk, and that is used for writing information on the magnetic disk and/or reading information from the magnetic disk.
2. Description of the Related Art
A magnetic head, such as a hard disk device, is arranged at an interval apart from a surface of a rotating magnetic disk, and functions to write information on the magnetic disk and/or to read information from the magnetic disk. In recent years, in association with high density recording on the magnetic disk, the interval between the magnetic head and the surface of the magnetic disk (a flying height of the magnetic head) has become extremely small. The interval is set at, for example, 10 nm or less. In such a case where the flying height of the magnetic head is very minute, when the flying height of the magnetic head is fluctuated for some reason or there is an unintentional projection on the surface of the magnetic disk, the magnetic head may contact the surface of the magnetic disk, thereby damaging the magnetic head. Therefore, in order to control the magnetic head so as not to contact the magnetic disk, it is desirable to control the flying height of the magnetic head and to preliminarily detect the presence or absence of the projection on the surface of the magnetic disk.
In Japanese Patent Laid-Open Publication No. 2004-164797, a projection detection head having a sensor for detecting a projection formed on a surface of a magnetic disk is disclosed. This sensor is arranged on an air bearing surface of a slider including the projection detection head, and detects a resistance change due to frictional heat generated by contact between the projection detection head and the magnetic disk. Since this configuration is configured with not only the magnetic head but also the projection detection head, the configuration is extremely complex, or two devices (a magnetic disk device and the projection detection device) are required. Therefore, this results in a complex configuration and an increase in cost.
In Japanese Patent Laid-Open Publication No. H8-167121, Japanese Patent No. 2,953,417, and Japanese Patent No. 2,980,074, another configuration is disclosed. In the configuration, a magneto-resistance (MR) element that actually works as a reproducing element of a magnetic disk device is used as a sensor for detecting a resistance change due to frictional heat generated by contact between a magnetic head and a magnetic disk. In Japanese Patent No. 2,953,417 and Japanese Patent No. 2,980,074 among the documents, the MR element is positioned in a stepped-back position from an air bearing surface that is opposite to the magnetic disk, and is connected to a heat conductive film exposed on the air bearing surface.
In the prior art, the presence or absence of the unintentionally formed projection (abnormal projection) on the surface of the magnetic disk is detected, and the sensor does not exercise a special functional effect when a normal magnetic disk is used on which an unintentional projection is not present.
For a sensor for detecting the resistance change due to the frictional heat generated by contact between the magnetic head and the magnetic disk, when such a sensor is away from a position where the magnetic head directly contacts the magnetic disk, it is sometimes impossible to detect the contact. However, it is necessary for the reproducing element of the magnetic disk device to be positioned in a suitable position for reproducing information. Accordingly, the reproducing element may be positioned at an unsuitable position as the sensor for detecting the contact. A tunnel magneto-resistance (TMR) element that has been the mainstream of a reproducing element in recent years is not appropriate when the flying height of the magnetic head is small as described above. That is because a change in an electrical resistance value of a barrier layer caused by a change in heat is small so that detection sensitivity is not excellent.
In U.S. Pat. No. 7,589,928, a device is disclosed for measuring a flying height of a magnetic head from the magnetic disk, i.e., an interval between the magnetic head and the surface of the magnetic disk regardless of the presence or absence of a projection on a surface of a magnetic disk. In this device, a configuration with a sensor is disclosed in addition to a recording element and a reproducing element of the magnetic head. The sensor is disposed for detecting a temperature change due to a change in the flying height of the magnetic head from the magnetic disk.
As described above, in order to detect a temperature change on the air bearing surface based on a resistance change of the sensor, it is desired that large resistance change easily occurs. Ordinarily, regarding a material such as metal, electrical resistance increases as the length along a direction where current flows increases. Also, it is known that large resistance change easily occurs. However, in the magnetic head, when a width in a track direction becomes wider by making the sensor larger, a range of sensing becomes larger. It makes difficult to accurately detect a position of a projection on the magnetic disk where each track width is minute due to progress in high density recording.
Specifically, the electrical resistance value of the sensor is obtained when voltage is applied between a pair of lead films to which both of edge parts in the track direction of the sensor are respectively connected and the sense current flows. When detecting the projection on the certain track on the magnetic disk, if the sensor which is wider in the track direction is used to make the detection of the resistance change easier, it may include an adjacent track to the track with a projection as a sensing area. As a result, sometimes, it is difficult to pinpoint a track with a projection. In order to detect minute projections in a specific track without affecting an adjacent track, it is desired to narrow a width in a track direction of the sensor. However, when the width in the track direction of the sensor is narrowed, an interval between a pair of lead films becomes smaller and the electrical resistance value of the sensor becomes smaller. As a result, to total electrical resistance value of all paths in which the sense current flows, a ratio of an electrical resistance value and a contact resistance of the lead films increases, a ratio of part (the electrical resistance value of the sensor) attributed to the resistance change decreases, and a detection sensitivity decreases. Therefore, it is difficult to accurately detect a position of minute projection having small frictional heat.